Settlement Deformation of Surface Buildings Caused by of Precipitation Pilot

Xie Siwei, Yang Yong, Wang Guihe*, Qian Feng

*Corresponding author: Wang Guihe, [email protected]

ABSTRACT construction using underground excavation tends to cause deformation. In general, precipitation pilot tunnels built during the process of subway construction are shallow, and are built in areas with a high concentration of buildings, which increases the likelihood of large deformations of surface buildings. In this paper, the finite element method is used to simulate and analyze the effects of tunnel construction adjacent to middle and low buildings (using overall ) by underground excavation. The research results show that buildings extend the influence range of ground settlement caused by tunnel excavation, and tunnel excavation also causes uneven settlement of adjacent buildings. Compared with strip foundation, raft foundation bears a larger building load and reduces the uneven settlement of buildings. With increasing depth, the buildings have an obvious influence on the curvature of the horizontal settlement of the ground surface. With increasing horizontal distance(S), the differential settlement of a building’s longitudinal axis decreases gradually, and the differential settlement of the horizontal axis first increases then decreases. When S = 8 m, that is, when S = 1 H, the differential settlement of the horizontal axis is the largest, and damage to building foundations is the most serious. The pre-reinforcement measures of large pipe shed + deep hole grouting for controlling building foundation settlement is better than the whole section grouting method, which effectively reduces the uneven settlement of building foundations caused by tunnel construction.

KEYWORDS: Tunnel excavation; Buildings; Numerical simulation; Land

INTRODUCTION

In the process of urban subway construction, the induced surface subsidence is a common phenomenon and a key issue in tunnel construction, which has attracted the attention of many researchers [1-4]. In order to study the deformation law of surface subsidence caused by subway construction, many researchers have carried out related researches [5-9], including the early empirical

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Vol. 24 [2019], Bund. 03 736 studies and theoretical analysis [10]. Peck simply estimated the displacement caused by tunnel construction by using Gaussian curves [11]. Chi et al. optimized the analysis of ground subsidence caused by tunnel excavation with the equivalent ground loss model and made a reasonable prediction on ground deformation caused by shield tunnel construction [12]. On-site monitoring research [13], physical model test [14] and numerical simulation analysis [15][16] enrich the study methods on this problem. Thayanan Boonyarak observed the ground deformation of Bangkok tunnel in construction period[17]; Lee and Bassett analyzed the interaction of two-dimensional pile-soil-tunnel based on model tests and numerical simulations, and conclude that the affected area is deeper and greater than the previous study [18].Mroueh and Shahrour discussed the effect of tunnels on adjacent pile foundations by using elastoplastic three-dimensional finite element software and reveal effect of pile group [19]. At present, the methods to research ground deformation caused by tunnels have their own advantages and disadvantages. The mutual verification of various methods can improve the reliability of evaluation on ground deformation. Taking the C# tunnel near Rome S. Giovanni station as an case study, Miliziano and Lillis made a comparative analysis between monitoring data and numerical simulation [20], Phienwej et al. developed a three-dimensional numerical analysis to evaluate the ground deformation caused by the EPB shield tunnel of Bangkok Metro [21], verified it with on-site monitoring, and concluded that it is feasible to analyze the ground deformation with three-dimensional numerical simulation analysis. On the prediction of ground subsidence considering the mutual restraint between buildings and strata, there are few researches. In this paper, in order to study the influence of urban shallow tunnel construction on adjacent buildings, a three-dimensional numerical model was built by FLAC3D[22]. And the specific stratum and shallow foundation of brick- structures was selected as research objects to reveal the settlement characteristics of buildings with different foundation structures and different spatial positional relationships and explore the effect of different pre-reinforcement measures. This work provided a reference and guide on the safety evaluation of subway construction.

INFLUENCING FACTORS OF TUNNEL UNDERCUT CONSTRUCTION ON BUILDINGS

Considering the general stratigraphic conditions in Beijing as an engineering case, the 3-dimensional numerical model has a length × width × height = 60 m × 60 m × 50 m. The tunnel section has a size of 6 m × 6 m, and the building foundation is simulated as a cuboid four-node physical unit with a size of 8 m × 12 m × 2 m. These models obeyed the Mohr-Coulomb failure criterion. The ground buildings are calculated as six-storey brick-concrete residential buildings, and the floor of each building is added to the building foundation according to a uniform load of 20 kPa. The initial lining and steel arch frame are simulated by a shell element. The parameters are: elastic modulus of 25,500 MPa, Poisson's ratio of 0.26, thickness of 0.25 m and gravity of 2500 kg/m3. A contact unit is arranged between the stratum and the building. The parameters are: normal modulus of 1630.3 MPa, shear modulus of 160.3 MPa, internal angle of 20°, cohesive force of 0.1 MPa, and tensile stress of Vol. 24 [2019], Bund. 03 737

0.001 MPa. The effects of tunnel excavation on the surface buildings are analyzed by considering different foundation forms, different tunnel depths, different horizontal distances between buildings and tunnel axes, and different angles between the long axis of the building and the tunnel axis. The tunnel-building base location is shown in Fig. 1..

(a) Model top view (b) Model profile Figure 1: Top view and profile of the model

(1) Impact analysis of different foundation forms The buried depth of the tunnel was taken as 8 m. The differential settlement and plastic zone considering different foundation forms (strip foundation, raft foundation) was analyzed. The basic material parameters are selected according to the design standard of masonry structure GB50003. For the elastoplastic consideration, the Mohr-Coulomb yield criterion is used. The specific parameters are shown in Table 1 below.

Table 1: Basic material parameter values of buildings Bulk Shear Internal Tensile Basic form modulus / modulus / density (kg/m3) friction (kPa) strength GPa GPa angle (°) (kPa) Strip 4.46 2.09 2100 38 180 180 foundation Seesaw 17.84 13.38 2400 48 540 900 foundation

The model z=50 is sliced to obtain the horizontal displacement contour, as shown in Fig.2. After excavation, the largest displacement was formed at the two long sides and four corners of the foundation, and the value is positive on the left long side and negative on the right long side. The stress Vol. 24 [2019], Bund. 03 738 of the base floor is compressive. The horizontal displacement difference of the J1 and J3 monitoring points is 1.76 mm, and the horizontal displacement difference of the J1 and J3 monitoring points of the raft foundation is 0.59 mm, which is about 1/3 of the strip foundation. This also shows that the raft foundation itself has a large amount of stiffness and is resistant to large compressive stress deformation and bending deformation.

(a) Raft foundation (b) Strip foundation Figure 2: Horizontal displacement of the surface at the end of excavation in different basic forms

As shown in Fig.3, the maximum differential settlement of the vertical axis of the strip foundation is 12.46 mm and the maximum inclination was 1.04‰. The maximum differential settlement of the raft foundation is 8.89 mm, and the maximum inclination is 0.74‰, which was reduced by 28.7% compared with the strip foundation.

Figure 3: Differential settlement curves of different basic forms based on the vertical axis Vol. 24 [2019], Bund. 03 739

(2) Analysis of different horizontal distance effects

Figure 4: Different horizontal distance settlement curves between the vertical axis of the building and the middle line of the tunnel

Fig. 4 and Fig. 5 shows that with an increase of distance from the middle line of the tunnel (S), the surface settlement caused by tunnel excavation decreases, the settlement curve of the building foundation and the adjacent stratum is relatively flat, and the differential settlement of the vertical axis of the building is gradually reduced. The horizontal axis is also gradually reduced. The process of differential settlement is first to increase, then decrease. When S=0, the settlement of the building foundation and the differential settlement in the vertical axis direction are the largest, reaching 18.88 mm with an inclination of 2.38‰, while the differential settlement in the horizontal axis direction is almost zero. When S=H, the maximum differential settlement of the longitudinal axis of the building is only 3.14 mm, and the maximum differential settlement of the horizontal axis reaches 16.24 mm, with the inclination reaching 2.03‰. When the horizontal distance S>1.5 H, the basic settlement curve becomes convex. After that, the curvature of the settlement curve gradually decreases. When S>2 H, the difference between the horizontal axis and the vertical axis of the building foundation changes very little. The foundation slope is 0.12‰, and the building is greatly affected by the uneven settlement of the surface. Vol. 24 [2019], Bund. 03 740

(a) Vertical axis differential settlement curve (b) Horizontal axis differential settlement curve Figure 5: Differential settlement curves of building foundations at different horizontal distances

As shown in Table 2 and Fig. 6, by analyzing the horizontal displacement of the monitoring points J6 and J8 of the building foundation, the deformation curve of the displacement difference L at different horizontal distances is obtained. When L>0, the building foundation is under pressure. When L<0, when the foundation is in the tensile state, and when the building is at the centerline of the tunnel, the base floor is most stressed. When S=8 m, that is, S=1 H, the foundation of the building is the most severely affected. As the distance increases, the horizontal displacement difference L of the building base gradually approaches zero.

Table 2: Horizontal displacement statistics of buildings at different horizontal distances Different horizontal 0 m 4 m 8 m 12 m 16 m distance J6 displacement 7.82E-04 -2.59E-03 -3.95E-03 -7.83E-04 1.31E-03 value / m J8 displacement -6.69E-04 -3.68E-03 -3.37E-03 -5.97E-04 1.43E-03 value / m J8 displacement 1.45E-03 1.10E-03 -5.80E-04 -1.86E-04 -1.19E-04 value / m Vol. 24 [2019], Bund. 03 741

Figure 6: Horizontal displacement curve of the building foundation at different horizontal distances

(3) Analysis of the influence of different longitudinal axes

Figure 7: Different angle settlement curves at S=8 m Vol. 24 [2019], Bund. 03 742

Figure 8: Differential settlements of the vertical axis between different angles

The analysis in Fig. 7 shows that the maximum settlement below the building gradually increases with the increase of A at S=8 m, and the maximum settlement is 18.7 mm when A=90°. At A=45°, the differential settlement reached a maximum of 14.9 mm, at which time the building was in the most ∠ ∠ ∠ unfavorable state. Fig. 8 shows that the angle between the building and the tunnel axis has a great influence on the differential settlement of the vertical axis of the building. As angle A of the axis increases, the maximum differential settlement in the direction of the longitudinal axis of the building shows a process of first increasing, then decreasing.

CASE ANALYSIS

Taking the Wang-Wang interval precipitation tunnel of the No. 01 section of Beijing Metro Line 8 as an engineering case, numerical simulations of two pre-reinforcement methods, the "full-section deep hole grouting method" and "large pipe shed + deep hole grouting method", were finished. The section of the tunnel is gate shape with length × height = 5.5 m × 7.6 m. The model size is length × width × height = 50 m × 80 m × 50 m. The geometric model is shown in Fig. 9. The boundary conditions are as follows: constraints are imposed on the perimeter and bottom, and the upper part of the model is free. The initial ground stress only considers the self-weight stress of the soil and the self-weight of the existing building, and the self-weight of the building is calculated according to the uniform load of 20 kN/m2. In the calculation model, the soil material properties of each layer are considered according to homogeneous elasto-plasticity. The surrounding rock is a homogeneous and isotropic continuous medium, and the Mohr-Coulomb yield criterion is adopted. Vol. 24 [2019], Bund. 03 743

building

precipitation pilot tunnel

Figure 9: Schematic diagram of the relative position geometry model

According to the project’s engineering investigation report, the relative physical and mechanical parameters of the stratum in the study area are obtained. To simplify the calculation, the soil layers with similar properties in the lower part of the tunnel are combined and calculated. At the same time, the parameters not specified in the survey report are obtained according to regional norms and engineering experience in Beijing. The mechanical calculation parameters of each layer of soil are shown in Table 3.

Table 3: Physical and mechanical parameters of soil layers Interna Modulus Thick l Density of Poisson's Cohesion Material name ness friction (kg/m3) elasticit ratio (KPa) (m) angle y (MPa) (°) ① miscellaneous fill 5 1650 7.5 0.3 10 12.5 ② silty 9 1980 12.6 0.3 15 22.57 ③ pebbles, 7 2000 25 0.26 0 40 ④ silty clay 7 2000 35 0.25 20 22.5 ⑤ pebbles, gravel 5 2150 60 0.26 0 40 ⑥ fine , 5 2080 40 0.2 5 32 medium sand ⑦ silty clay 5 2020 8 0.25 25 2.5 ⑧ silty clay 7 2040 12.5 0.25 20 22.5

Vol. 24 [2019], Bund. 03 744

(1) Analysis of surface lateral section settlement The A-A section of the model in the longitudinal direction (y=32 m) (a section in front of the building) and the B-B section of y=47 m (the section directly below the building) was taken for analysis. Fig.10 shows that because of the eccentric effect of the building on the tunnel, the settlement value on the left side of the tunnel center line is larger than that of the right side, and that there is some difference in the settlement. Fig.11 shows that the maximum settlement of the foundation caused by the underground excavation of the precipitation guide tunnel is 13.9 mm, and that the large foundation shed + deep hole grouting pre-reinforcement method is used. The settlement is 7.14 mm, which is 6.26 mm less than the full-section grouting method, which is reduced by about 45%. It can be seen that the pre-reinforcement measures of the large pipe shed + deep hole grouting can effectively support the upper soil of the tunnel and reduce the ground subsidence and differential settlement.

Figure 10: Comparison diagram of the A-A and B-B section surface settlement

Figure 11: Comparison of the surface settlement of different pre-reinforcement methods

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(2) Analysis of the clearance of the leading hole Fig.12 shows that in the case of pipe shed+deep hole grouting pre-reinforcement, the horizontal convergence value of the right side of the B-B cross-section tunnel is 11.7 mm, and the left side is 16 mm. This reflects the bias of the surface building to the tunnel during the excavation of the precipitation guide hole. The effect of the clearance in the full-section grouting is only about 8 mm, which is about 50% lower than that of the large-tube grouting pre-reinforcement measures. It can be seen that full-section grouting can significantly reduce the horizontal convergence value of the undercut tunnel.

(a) Full-section grouting (b) Pipe shed grouting Figure 12: Comparison of the horizontal convergence of rainfall guide hole with different pre-reinforcement methods

(3) Settlement of building foundation The settlement curves of the two monitoring points J2 and J7 in the longitudinal direction of the building are shown in Fig. 13. The analysis shows that the differential settlement of the longitudinal axis of the building under the full-section grouting condition is greater than the differential settlement under the pipe shed grouting condition. The maximum settlement of the building occurs after the lining is completed, and the maximum differential settlement of the building occurs during the construction of the underpass building, at which time the building is in the most unfavorable condition. When the lining and construction are completed, the foundation settlement of the building gradually becomes stable.

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(a) Pipe shed grouting (b) Full-section grouting Figure 13: Diachronic settlement curve of the J2 and J7 monitoring points in the vertical axis of the building

Fig.14 shows that the maximum differential settlement in the longitudinal direction of the building occurs during the tunnel excavation in the immediate vicinity of the building. After the tunnel was excavated to y=30 m, the construction of the pipe shed was started, and a 50-m long pipe shed was applied at one time. When the tunnel is excavated to y=40 m, it will reach the base of the building. The profile view is shown in Fig. 13.

Figure 14: Longitudinal sectional view of the position of the building and tunnel (excavated to the fourth cycle)

After the completion of the five cycles of construction affected by the construction, the maximum settlement and maximum differential settlement of the building are compared, as shown in Fig.15(a) and (b). After analysis, it can be seen that under the pre-reinforcement measures of the large pipe shed grouting, the maximum differential settlement of the longitudinal direction of the building foundation is 3.8 mm, and the differential settlement caused by the full-section grouting reinforcement is 10.7 mm, which is about three times that of the pipe shed. Vol. 24 [2019], Bund. 03 747

(a) Maximum settlement of buildings

(b) Comparison of the maximum differential settlement of buildings Figure 15: Settlement and tilt comparison of buildings under different reinforcement conditions Vol. 24 [2019], Bund. 03 748

CONCLUSION

The strip-shaped foundation has relatively poorer ability of resisting deformation than the seesaw foundation. With the increase of horizontal distance, the differential settlement in horizontal direction firstly increases and then decreases. The results of numerical simulation of full-section grouting and large pipe shed+deep hole grouting methods show that when there is no building in the upper part of the tunnel, full-section grouting is better. When the buildings existed, the large pipe shed+deep hole grouting are better than full-section grouting method, which effectively reduces the maximum settlement and uneven settlement.

REFERENCES

[1] Yanjuan Hou,Qian Fang,Dingli Zhang,Louis Ngai Yuen Wong. Excavation failure due to pipeline damage during shallow tunnelling in soft ground[J]. Tunnelling and Underground Space Technology incorporating Trenchless Technology Research,2015,46. [2] Junhui Luo,Linchang Miao. Research on dynamic creep strain and settlement prediction under the subway vibration loading[J]. SpringerPlus,2016,5(1). [3] F. Pinto, A.J. Whittle. Ground movements due to shallow tunnels in soft ground. I: Analytical solutions[J].Journal of Geotechnical and Geoenvironmental Engineering, 140 (4) (2014). [4] X. Xie, Y. Yang, M. Ji Analysis of ground surface settlement induced by the construction of a large-diameter shield-driven tunnel in Shanghai, China[J].Tunnelling and Underground Space Technology, 51 (2016), pp. 120-132. [5] Y.S. Fang, S.J. Lin, J.S. Lin. Time and settlement in EPB shield tunneling[J]. Tunnels and Tunnelling, 25 (11) (1993), pp. 27-28. [6] S. Miliziano, A. de Lillis. Predicted and observed settlements induced by the mechanized tunnel excavation of metro line C near S. Giovanni station in Rome[J]. Tunnelling and Underground Space Technology, 86 (2019), pp. 236-246. [7] M. Migliazza, M. Chiorboli, G.P. Giani.Comparison of analytical method, 3D finite element model with experimental settlement measurements resulting from the extension of the Milan underground[J].Computers and Geotechnics, 36 (1) (2009), pp. 113-124. [8] Li Yuan, Zhen-Dong Cui,Jun Tan. Numerical simulation of longitudinal settlement of shield tunnel in the coastal city Shanghai[J]. Marine Georesources & Geotechnology, 2017,35(3). Vol. 24 [2019], Bund. 03 749

[9] S. Miliziano, A. de Lillis. Predicted and observed settlements induced by the mechanized tunnel excavation of metro line C near S. Giovanni station in Rome[J]. Tunnelling and Underground Space Technology, 86 (2019), pp. 236-246. [10] M. Huang, C. Zhang, Z. Li. A simplified analysis method for the influence of tunneling on grouped pilesTunnelling and Underground Space Technology.[J]. 24 (4) (2009), pp. 410-422 [11] Peck, R.B., Deep excavations and tunnelling in soft ground. In: Proceedings of the 7th International Conference on and Foundation Engineering, Mexico City, (1969)pp. 225–290. [12] Shue-YeongChi ， Jin-ChingChern ， Chin-Cheng Lin. Optimized back-analysis for tunneling-induced ground movement using equivalent ground loss model. Tunnelling and Underground Space Technology.[J]. 16(3)(2001),pp.159 - 165 [13] R. Mair, R. Taylor, A. Bracegirdle. Subsurface settlement profiles above tunnels in clays. Geotechnique.[J] 43 (2) (1993) [14] Y.-J. Lee, R.H. Bassett. A model test and numerical investigation on the shear deformation patterns of deep wall-soil-tunnel interaction .Canadian Geotechnical Journal.[J].43 (12) (2006), pp. 1306-1323. [15] H. Liu, J.C. Small, J.P. Carter. Full 3D modelling for effects of tunnelling on existing support systems in the Sydney region.Tunnelling and Underground Space Technology.[J]. 23 (4) (2008), pp. 399-420 [16] P. Jongpradist, T. Kaewsri, A. Sawatparnich, S. Suwansawat, S. Youwai, W. Kongkitkul, J. Sunitsakul Development of tunneling influence zones for adjacent pile foundations by numerical analyses. Tunnelling and Underground Space Technology.[J]. 34 (2013), pp. 96-109. [17] T. Boonyarak, K. Phisitkul, C.W. Ng, W. Teparaksa, Z.Z. Aye. Observed ground and pile group responses due to tunneling in Bangkok stiff clay . Canadian Geotechnical Journal. [J], 51 (5) (2014), pp. 479-495. [18] Y.-J. Lee, R.H. Bassett. Influence zones for 2D pile-soil-tunnelling interaction based on model test and numerical analysis. Tunnelling and Underground Space Technology.[J]. 22 (3) (2007), pp. 325-342 [19] H. Mroueh, I. Shahrour. Three-dimensional finite element analysis of the interaction between tunneling and pile foundations. International Journal for Numerical and Analytical Methods in Geomechanics.[J] 26 (2002), pp. 217-230. [20] Salvatore Miliziano, Armando de Lillis. Predicted and observed settlements induced by the mechanized tunnel excavation of metro line C near S. Giovanni station in Rome. Tunnelling and Underground Space Technology.[J].86 (2019), pp. 236-246. Vol. 24 [2019], Bund. 03 750

[21] N.Phienwej, C.P.Hong, A.Sirivachiraporn. Evaluation of ground movements in EPB-shield tunnelling for Bangkok MRT by 3D-numerical analysis. Tunnelling and Underground Space Technology.[J].21（3-4） (2006), pp.273.

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Editor’s note. This paper may be referred to, in other articles, as: Xie Siwei, Yang Yong, Wang Guihe*, Qian Feng: “Settlement Deformation of Surface Buildings Caused by Construction of Precipitation Pilot Tunnels” Electronic Journal of , 2019 (24.03), pp 735-750. Available at ejge.com.